/Engine/Source/Runtime/ImageCore/Public/ImageCoreUtils.h
Returns the number of mips that constitute a full mip chain for the given top level mip size. InVolumeZ is ignored unless bInIsVolume is true
namespace FImageCoreUtils { int32 FImageCoreUtils::GetMipCountFromDimensions ( int32 InSizeX, int32 InSizeY, int32 InVolumeZ, bool bInIsVolume ) }